Not applicable.
Investigators in the electric motor arts have been called upon to significantly expand motor technology from its somewhat static status of many decades. Improved motor performance particularly has been called for in such technical venues as computer design and secondary motorized systems carried by vehicles, for example, in the automotive and aircraft fields. With progress in these fields, classically designed electric motors, for example, utilizing brush-based commutation, have been found to be unacceptable or, at best, marginal performers.
From the time of its early formation, the computer industry has employed brushless d.c. motors for its magnetic memory systems. The electric motors initially utilized for these drives were relatively expensive and incorporated a variety of refinements particularly necessitated with the introduction of rotating disc memory. For example, detent torque phenomena has been the subject of correction. The phenomena occurs as a consequence of the nature of motors configured with steel core stator poles and associated field windings performing in conjunction with permanent magnets. With such component combinations, without correction, during an excitation state of the motor windings which create motor drive, this detent torque will be additively and subtractively superimposed upon the operational characteristics of the motor output to distort the energized torque curve, increase ripple torque, reduce the minimum torque available for starting and, possibly develop instantaneous speed variations. Such instantaneous speed variations generally have not been correctable by electronics. Particularly over the recent past, the computer industry has called for very low profile motors capable of performing in conjunction with very small disc systems and at substantially elevated speeds.
Petersen, in U.S. Pat. No. 4,745,345, entitled xe2x80x9cD.C. Motor with Axially Disposed Working Flux Gapxe2x80x9d, issued May 17, 1988, describes a PM d.c. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed xe2x80x9caxiallyxe2x80x9d wherein the transfer of flux is parallel with the axis of rotation of the motor. This xe2x80x9caxialxe2x80x9d architecture further employs the use of field windings which are simply structured, being supported from stator pole core members, which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and efficiency and, ideally, may be designed to assume very small and desirably variable configurations.
Petersen in U.S. Pat. No. 4,949,000, entitled xe2x80x9cD.C. Motorxe2x80x9d, issued Aug. 14, 1990 describes a d.c. motor for computer applications with an axial magnetic architecture wherein the axial forces which are induced by the permanent magnet based rotor are substantially eliminated through the employment of axially polarized rotor magnets in a shear form of flux transfer relationship with the steel core components of the stator poles. The dynamic tangentially directed vector force output (torque) of the resultant motor is highly regular or smooth lending such motor designs to numerous high level technological applications such as computer disc drives which require both design flexibility, volumetric efficiency, low audible noise, and a very smooth torque output.
Petersen et al, in U.S. Pat. No. 4,837,474 entitled xe2x80x9cD.C. Motorxe2x80x9d, issued Jun. 6, 1989, describes a brushless PM d.c. motor in which the permanent magnets thereof are provided as arcuate segments which rotate about a circular locus of core component defining pole assemblies. The paired permanent magnets are magnetized in a radial polar sense and interact without back iron in radial fashion with three core components of each pole assembly which include a centrally disposed core component extending within a channel between the magnet pairs and to adjacently inwardly and outwardly disposed core components also interacting with the permanent magnet radially disposed surface. With the arrangement, localized rotor balancing is achieved and, additionally, discrete or localized magnetic circuits are developed with respect to the association of each permanent magnet pair with the pole assembly.
Petersen in U.S. Pat. No. 5,659,217, issued Feb. 10, 1995 and entitled xe2x80x9cPermanent Magnet D.C. Motor Having Radially-Disposed Working Flux-Gapxe2x80x9d describes a PM d.c. brushless motor having outstanding performance characteristics which is producible at practical cost levels commensurate with the incorporation of the motors into products intended for the consumer marketplace. These motors exhibit a highly desirable heat dissipation characteristic and provide improved torque output in consequence of a relatively high ratio of the radius from the motor axis to its working gap with respect to the corresponding radius to the motors"" outer periphery. The torque performance is achieved with the design even though lower cost or, lower energy product permanent magnets may be employed with the motors. See also: Petersen, U.S. Pat. No. 5,874,796, issued Feb. 23, 1999.
Over the years of development of what may be referred to as the Petersen motor technology, greatly improved motor design flexibility has been realized. Designers of a broad variety of motor driven products including household implements and appliances, tools, pumps, fans and the like as well as more complex systems such as disc drives now are afforded a greatly expanded configuration flexibility utilizing the new brushless motor systems. No longer are such designers limited to the essentially xe2x80x9coff-the-shelfxe2x80x9d motor variety as listed in the catalogues of motor manufacturers. Now, motor designs may become components of and compliment the product itself in an expanded system design approach.
During the recent past, considerable interest has been manifested by motor designers in the utilization of processed ferromagnetic particles in conjunction with pressed powder technology as a substitute for the conventional laminar steel core components of motors. With this technology, the fine particles which are pressed together essentially are mutually electrically insulated. So structured, when utilized as a motor core component, the product will exhibit very low eddy current loss which will represent a highly desirable feature, particularly as higher motor speeds and resultant core switching speeds are called for. As a further advantage, for example, in the control of cost, the pressed powder assemblies may be net shaped wherein many intermediate manufacturing steps and quality considerations are avoided. Also, tooling costs associated with this pressed powder fabrication are substantially low as compared with the corresponding tooling required with typical laminated steel fabrication. The desirable molding approach provides a resultant magnetic particle structure that is 3-dimensional magnetically and avoids the difficulties encountered in the somewhat two-dimensional magnetic structure world of laminations. See generally U.S. Pat. No. 5,874,796 (supra).
The high promise of such pressed power components, however, currently is compromised by the unfortunate characteristic of the material in exhibiting relatively low permeability as contrasted at least with conventional laminar core systems. Thus the low permeability has called for 1xc2xd to 2 times as many ampere turn deriving windings. In order to simultaneously achieve acceptable field winding resistance values, the thickness of the winding wire must be increased such that the wire gauge now calls for bulksome structures which, in turn, limit design flexibility. Indeed, designers confronting the permeability values available with processed ferromagnetic particle technology will, as a first inclination, return to laminar structures. This is particularly true where control over the size of the motors is mandated as, for example, in connection with the formation of brushless d.c. motors employed with very miniaturized packaging . However, the disc drive industry now seeks such compact packaging in conjunction with high rotational speeds. In the latter regard, speed increases from around 7200 rpm to 15000 rpm and greater now are contemplated for disc drives which, in turn, must perform with motors the profile of which is measured in terms of a small number of millimeters. In general, lamination-based core structures cannot perform as satisfactorily at the higher core switching speeds involved, while particulate core-based structures are defeated by the size restraints.
The present invention is addressed to PM d.c. motors having stator core assemblies formed of processed ferromagnetic particles and created by molding or pressing procedures. Practical configurations for these motors are developed with architectures which accommodate for the lower permeability characteristics of their stator forming material through the application of a higher level of P.M. induced fields and larger cross sectional area of the winding support region to achieve the required performance comparable to conventional laminated stator core structures. To make optimum use of the form pressed particulate material, a stator assembly shape is provided which maximizes the efficiency of coupling of the permanent magnet field into each stator core component, as well as improves the coupling efficiency of the applied field at the field winding support region. The latter coupling efficiency is developed through the utilization of ramp-shaped transitions from one level at an induction region to a next adjacent level at the field winding support region of each core component of the stator core assembly. Efficiency further is realized by adherence to a criteria wherein the widthwise extent or width of each core component of the stator assembly at the field winding region and the arcuate or circumferential length of the flux interaction surface at each induction region meet a requirement wherein their ratio cannot exceed about 2.5. Enhanced induction of the permanent magnet field at the induction region of each core component is evolved by an enlarged face length or length of the flux confronting surface of each core component taken in parallel with the motor axis. That length will be about coextensive with the corresponding length of the permanent magnet assembly in the motor rotor.
By meeting these design criteria, motors can be designed which meet severe size and/or shape limitations and which may take advantage of the lower eddy current losses evidenced by pressed processed ferromagnetic particulate materials. The resultant stator cores will sustain very high field switching rates without otherwise unacceptable losses, which typically are manifested in excessive and generally unacceptable heat development.
In one approach of the invention, further advantage is taken of these processed ferromagnetic particulate structures in that the stator core assemblies can be formed of discrete core components which are interkeyed to form a plurality of core components interconnected by a back iron assembly formed of the same type material. Alternately the back iron assembly can be formed from stamped laminations albeit with some accompanying increase in losses at higher switching frequencies. Compression stamped laminations Tensioning assemblies are utilized in retaining these assemblies together and further, they can be interkeyed and assembled together with adhesives.
Another feature of the invention is to provide a d.c. motor exhibiting a predetermined torque constant, field winding resistance and functioning air gap radius extending from a motor axis. The motor includes a rotor having a sequence of generally arcuate regions of predetermined magnetization and confronting magnetic surface of principal dimension in parallel with the motor axis. This confronting magnetic surface is located in correspondence with the air gap radius and is rotated about the motor axis. A stator core assembly is provided having spaced core components formed of the noted pressure shaped processed ferromagnetic particles which are generally mutually insulatively associated. Each core component of the stator core assembly is disposed about a radius extending from the motor axis and has a flux interaction surface located adjacent the rotor confronting magnetic surface to define a functioning air gap. The flux interaction surface has a face length parallel with the motor axis and a face width selected to provide a magnetic field coupling induction acting with a selected core component winding support region area and winding turns that corresponds with the predetermined torque constant and field winding resistance. Each core component has the winding support region radially spaced from and in flux transfer communication with the flux interaction surface and has a flux interacting pole region which exhibits a cross-sectional area effective for conveying of confronting magnetic flux and coil generated electromagnetic flux without saturation. The stator assembly includes a back iron assembly formed of pressure shaped processed ferromagnetic particles which are generally mutually insulatively associated. This back iron assembly is radially spaced from and in flux transfer association with each core component winding support region and has cross-sectional area attributes effective for magnetic flux conveyance without saturation. The noted core component flux interaction surface face width has a value less than about 2.5 times the winding region width. A field winding assembly is provided which is configured to exhibit the predetermined field winding resistance and which includes winding components located at each core component, extending in electromagnetic flux coupling relationship about the winding support region, the winding components being controllably electrically excitable for effecting driven rotation of the rotor about the motor axis.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises the method, system and apparatus possessing the construction, combination of elements, arrangement of parts and steps which are exemplified in the following detailed description.
For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection with the accompanying drawings.